1. Field of the Invention
The present invention relates to a speed reducer for a vehicle and, more specifically, to a speed reducer for a vehicle that: decreases the temperature of a worm wheel by conducting, to a boss, frictional heat generated when the worm wheel is rotated by the rotational force of a worm shaft caused by driving a motor, thereby preventing expansion of the worm wheel and enabling a worm and the worm wheel to be normally engaged with each other to prevent power loss and to stably provide a steering assist force for a driver; and transfers the frictional heat between the worm and the worm wheel to the boss to decrease the temperature of the worm wheel, thereby enhancing the durability of the worm wheel and preventing noise to make a driver feel comfortable when transmitting a rotational force from the worm shaft to the worm wheel.
2. Description of the Prior Art
FIG. 1 is a sectional view illustrating the internal structure of a speed reducer for a vehicle in the related art.
The speed reducer has a structure in which: a worm shaft 104 made of metal, such as steel, has a worm 102 formed on the middle part thereof; worm shaft bearings 106 are mounted on opposite ends of the worn shaft 104 to support the worm shaft 104; and the worm shaft 104 and a motor shaft 108 are connected with each other such that the worm shaft 104 is rotated by driving a motor 110.
A worm wheel 114 made of, for example, a resin composite is provided at one side of the worm 102 as a reduction gear so as to be engaged with the worm 102 formed on the middle part of the worm shaft 104. The worm wheel 114 is mounted on a steering shaft 112 that transmits the rotational force of a steering wheel (not illustrated) operated by a driver such that the rotational force of the worm shaft 104 generated by driving the motor 110 is transmitted to the steering shaft 112. The worm wheel 114 and the worm shaft 104, which are engaged with each other in a worm gear manner, are mounted within a housing 116 for protection from the outside.
An electronic control unit (not illustrated) provided in the vehicle controls the driving of the motor 110 according to the vehicle's driving conditions, and the rotational force of the worm shaft 104 generated by driving the motor 110 is added to the rotational force of the steering wheel operated by the driver and is transmitted to the steering shaft 112, thereby making the driver's steering smooth and stable.
Herein, the worm wheel of the speed reducer serves to reduce the RPM of the motor and to transmit the same to the steering shaft, and the gear made of a resin composite may be paired with the gear made of metal, as described above.
However, the speed reducer for a vehicle in the related art has a problem in that the worm wheel is rotated by the rotational force of the worm shaft generated by driving the motor so that the temperatures of the worm shaft and the worm wheel may increase due to the continuous friction therebetween and, in particular, the worm wheel formed of a resin composite (such as plastic, etc.) expands on account of the frictional heat so that the worm cannot be normally engaged with the worm shaft and power loss may be caused, whereby a steering assist force cannot be stably provided for a driver.
In addition, when the worm wheel expands due to the frictional heat between the worm shaft and the worm wheel, wear occurs between the worm shaft and the worm wheel so that durability may be weakened, and when a rotational force is transmitted from the worm shaft to the worm wheel, noise may cause a driver to feel uncomfortable.